Although commercial application of ultrasonic holography has been actively pursued by many persons in the scientific and industrial communities for many years, only limited results have been obtained even though it was once thought that ultrasonic holography held great promise. It was felt that the application of ultrasonic holography was particularly applicable to the fields of non-destructive testing of materials and medical diagnostics of soft tissues that are relatively transparent or translucent to ultrasonic radiation. One of the principal problems that has been encountered and not effectively resolved is the difficulty of obtaining visible results having high resolution content.
Solutions to this problem have been elusive, in part because of the difficulty in identifying the many causes that contribute to the problem. One culprit that is believed to materially contribute to the problem has been the difficulty of generating undistorted ultrasonic plane waves from a large surface piezoelectric transducer. It has been suggested that "edge effect" radiation from the side and edges of the piezoelectric wafer materially interferes with and adversely affects the ability of the transducer to generate undistorted plane waves for insonifying the subject object. To illustrate this point, reference is made to a typical prior art ultrasonic holography system that is schematically shown in FIGS. 1 and 2.
Such a typical "real time" ultrasonic holographic system is generally identified in FIG. 1 with numeral 10. The system 10 is intended to inspect the interior of an object 12. The system 10 generally has a hologram generating sub-system 13 and a hologram viewing sub-system (optical sub-system) 32. One of the principal components and the main subject of the focus of this invention is the provision of ultrasonic transducers, generally referred to as the object transducer 14 for generating ultrasonic plane waves 16 for insonifying the object 12 and reference transducer 22 for generating an off-axis beam.
The ultrasonic energy transmitted through the object 12 is directed to a hologram detection surface 18, which is generally an area of a liquid-gas interface or liquid surface, such as a water surface. Generally the hologram detection surface 18 is physically isolated in a detection container 20 to minimize distortions caused by vibration. The ultrasonic reference transducer 22 generates an off-axis ultrasonic beam that is also directed to the hologram detection surface 18 to form a standing hologram. It is frequently desirable to pulse the transducers 14 and 22 at desired intervals to minimize dynamic distortions of the detector surface 18.
Generally an ultrasonic lens assembly 26 is utilized to provide a focused hologram of a desired plane 27 within the object 12. In the example shown, the assembly 26 has a stationary lens 28 having a focal length coincident with the plane of the hologram detection surface 18. A movable complementary lens 30 is provided to be moved to focus on the desired object plane 27 of the object 12.
The optical subsystem 32 includes a source of coherent light, preferably a laser 34 for generating a beam of coherent light. The laser light beam is directed through a laser lens 36 to achieve a point source that is located at or near the focal point of a collimating lens 38 and then onto the hologram detector surface to illuminate the hologram. The reflected coherent light radiation containing holographic information is directed back through the optical lens 38 and separated into precisely defined diffracted orders in the focal plane of the collimating lens 38. A filter 42 is used to block all but a first order pattern 44 for "real time" observation by a human eye 46 or an optical recorder, such as a video recorder.
As illustrated in FIG. 1, the prior art ultrasonic transducers, in addition to generating plane waves 16, generate edge effect waves 48 that adversely interfere with the fidelity of the plane waves 16 which causes a reduction in the resolution and clarity of the produced hologram. FIG. 2 illustrates the distortions in the plane waves. FIG. 2 illustrates the energy profile 52 of the plane wave emanating from the front face of the transducer 14. The energy profile or curve 52 has dramatic end or edge curve sections 54 showing the sharp decrease in the power levels at the edges of the transducer. The curve 52 also shows an irregular and distorted central plateau 60 of the wave form indicating the adverse interference of the edge effect waves distorting the plane waves emanating from the front face of the transducer.
A principal objective of this invention to provide an ultrasonic transducer that materially reduces the generation of disruptive edge effect sound waves. The present invention more nearly operates closer to the more ideal condition illustrated in FIG. 3, having a power wave form distribution across the face of the transducer with a uniform, undistorted central section 60 with gradually decreasing transition segments 64 and 66 toward the transducer edges.
These and other objects and advantages of the present invention will become apparent upon reading the following description of the preferred and alternate embodiments of the present invention.